The purpose of heat treatment of metal product is to develop the full effects of the various elements in metal product, such as steel, as related to desired properties, through structural or phase changes. Rods and bars vary in hardness and microstructure in relation to the chemical composition of the metal and, therefore usually require some form of heat treatment to obtain a physical condition best suited for the final product. Low-and medium-carbon bars often are used in the as-rolled condition, but higher carbon steels and most alloy steels require heat treatment. This treatment consists of some form of annealing, normalizing, or quenching and tempering or a combination of treatments. By thermal treatment, it is possible to obtain the transformation of a particular metal, like a particular steel, to obtain the desired mechanical properties.
The annealing process is a function of time and temperature. The time and temperature depend on the grade and quality of metal being produced. Annealing and spheroidizing annealing of rod and bar is accomplished using one of two basic furnace designs. The first type of design is called "batch" heat-treatment, and as the name implies, the product is thermally treated as a unit or batch. The second design is termed "continuous type furnaces," and these are usually known for higher productivity and improved uniformity of treatment of the metal product. The invention herein is a continuous annealing furnace. The furnace, however, can be used in "batch" modality, for instance when working with a limited productivity on the line for the heat treatment using the annealing furnace or on the bar line in general.
Once a type of thermal treatment has been established for a given product, the treatment time is relatively fixed. The capacity of a furnace for metal product is determined by the charge weight and the cycle rate. Usually, both the charge weight and the cycle rate are fixed for a particular type of thermal treatment. Annealing is characterized by faster cycle rates and lower product ductility, while spheroidization is a slower process and produces higher product ductility. Total furnace capacity is therefore the result of the mix of annealing and single spheroidizing cycles. As the volume of spheroidization increases, the furnace capacity decreases.
Some capacity improvement has been obtained by the manipulation of the temperatures within the process, allowing for a shorter cycle time. Once optimum time and temperature relationships have been established for a particular thermal treatment, rates of transporting the metal product through the furnace can generally be considered as fixed.
The typical annealing process for rods and bars of round, square, and rectangular shapes is as follows: Conventionally, bars of round, square and hexagonal cross sections varying from 15 mm to 100 mm in diameter and from 3 mm to 100 mm thick and up to 100 mm wide for rectangular cross sections may be treated in such a furnace by being heated in a range of approximately 300.degree. C. to 1100.degree. C. while being conveyed through the annealing furnace. There exists a need for conveyors capable of conveying metal product of a variety of cross sections such as squares, rounds, rectangles, etc. without particular adaptation for each and a need for continuous conveyors capable of delivering metal product aligned with the next in-line process.
Precise control of the temperature of the rod or bar during the annealing process requires the temperature profile in the annealing furnace to be uniform over the volume enclosing the metal product. In larger conventional box furnaces such uniform temperature profiles are difficult to achieve and may exist in only a small fraction of the total volume of the heated furnace chamber.
U.S. Pat. No. 2,139,067 issued to J. J. Boax in 1938 describes a heating furnace. The heating furnace is arranged to receive articles in groups or batches and to move the received batch through the heating chamber at a speed correlated to the temperature so that the batch of articles will be delivered at the exit of the chamber at the desired temperature. This system is designed primarily for batch handling of metal product, the heating of the metal bars on the batch conveyor may not be even and batch delivery of metal product to the furnace requires either that the furnace to be off-line or the furnace have some type of hopper to collect articles in batch.
Many prior art annealing furnaces are off-line annealing furnaces. Because of the size and time requirements of prior art annealing furnaces they are usually not included in-line on a steel making line, for example. The prior art method of thermal treatment of metal product in off-line processes normally requires long cycle times, which leads to very low productivity levels, high heat treatment costs and less energy savings.
The present invention relates to an in-line continuous furnace for thermal treating long rods and bars which is flexible in function and efficient in operation. The present invention overcomes the disadvantages of the prior art and can obtain the following cycle times in a steel rolling line: for spheroidizing-annealing steel product can be treated for one to two hours at 680 to 720.degree. C.; for workability annealing steel product can be treated for 30 to 40 minutes at 650.degree. C. and for shearability annealing steel product may be treated for 30 to 40 minutes at 650.degree. C.